The present invention relates to NiAl-based intermetallic composites, and more particularly, to a new NiAlxe2x80x94CoCrAlY bond coat optionally having particulate AlN dispersed therein. The bond coat has particular application as part of a thermal barrier coating for metallic components used in high temperature applications.
Multilayer thermal barrier coatings on superalloy substrates are comprised of an intermetallic bond coat, a thermal grown oxide layer and a zirconia top coat that provides thermal protection. Known bond coats include CoCrAlY and NiCrAlY. These bond coats are alumina formers and provide oxidation resistance. However, because of the low aluminum content of these bond coat materials, their oxidation resistance is limited to shorter times and lower temperatures then desired in many applications. Further, their coefficient of thermal expansion mismatch with the zirconia thermal barrier coating causes rapid degradation.
In accordance with the present invention, a bond coat with improved long-term oxidation resistance and coefficient of thermal expansion compatibility with the thermal barrier coating is provided.
It has now been found that NiAl and CoCrAlY may be combined to provide improved bond coats. The performance of the bond coat may be further enhanced with the dispersion therein of particulate AlN.
AlN is believed to operate to enhance oxidation resistance by providing an aluminum source useful to form alumina scale. In addition to enhancing oxidation resistance, AlN has also been found to reduce the coefficient of thermal expansion of the resulting composite to more closely match that of the ceramic thermal barrier coat, e.g. zirconia. Accordingly, the resulting composite is characterized by increased oxidation resistance and thermal fatigue properties.
The NiAl and CoCrAlY alloy may include 15 to 30 volume percent CoCrAlY, the balance being NiAl. The NiAl may be at 50 to 55 atom percent.
The NiAlxe2x80x94CoCrAlYxe2x80x94AlN composite may comprise about 10 to 15 volume percent AlN, 15 to 30 volume percent CoCrAlY and the balance is NiAl. Good results have been obtained with about 10 volume percent AlN and 15 volume percent CoCrAlY, the remainder being NiAl.
A further improvement provided by the AlN particulate is increased mechanical strength. More particularly, the modulus of the resulting composite is increased.
The NiAlxe2x80x94CoCrAlYxe2x80x94AlN composite is lightweight, tough and highly creep resistant. The composite also has good thermal conductivity.
Cryomilling may be used in the preparation of the composite. More particularly, NiAl and CoCrAlY may be mixed and cryomilled in liquid nitrogen with the use of a grinding media. During the subsequent forming and heating of the composite, the AlN is formed as a particulate dispersion within the NiAlxe2x80x94CoCrAlY matrix.